It has heretofore been known to provide a booster steering control valve of the kind which utilizes a metering pump which receives high pressure from an engine driven pump to meter flow via a metering pump to a servomotor, responsive to being rotated by rotation of a vehicle steering wheel. In such construction the valve comprises a housing with a valve sleeve therein and various passages and grooves so that actuation of the valve sleeve by rotation of the vehicle steering wheel brings into selective connection various grooves and passages to control flow to and from the chambers of a double acting servomotor. It has also been known to provide a back pressure valve in a high pressure passage of the housing to which passage the servopump connects. Such back pressure valve is normally closed when the valve is in neutral position and pressure from a servopump is bypassed through the valve to a tank, but can open during steering to permit flow to the metering pump through the valve sleeve. The purpose of the back pressure valve is to insure that the pressure circuits for steering are "closed" under all circumstances including when the steering operation is taking place and prevents road roughness jolts on the vehicle wheels from being transmitted through the valve to the steering wheel.
In general, the clearance between the valve sleeve and the housing is very small and the pressure within the valve sleeve serves to maintain such clearance to a minimum for avoidance of leakage.
A control valve of the above description is shown in brochures entitled "Der Neue ZF-Servostat" published March 1980 and "ZF-Servostat 2" published by Zahnradfabrik Friedrichshafen AG in September of 1980, and identified as publication No. G 8470 D-SL. Other background information is found in U.S. Pat. Nos. 4,174,612; 4,193,471; and 4,258,752.
Due to the effect of driving with road surface forces upon the vehicle wheels, there can occur after completion of a steering operation, a partial pressure effect upon the valve sleeve while at the same time there is a considerable drop in delivery pressure from the servopump. This is caused by the fact that the interior pressure in the valve sleeve cannot be reduced since it is blocked at the back pressure valve from connection to the pressureless flow path of hydraulic oil in the neutral position of the valve. While the back pressure valve, being a non-return valve, prevents road jolts against the vehicle wheels from reaching the steering wheel, the blocking of a balancing pressure causes the partial pressure to occur within the valve sleeve. Since the usual working pressures of hydraulic booster systems are high the valve sleeve becomes deformed and can jam within the housing.